Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Such fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to complete the electrochemical reaction and form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to complete the electrochemical reaction and form liquid water as the reaction product.
In conventional fuel cells, the MEA is interposed between two fluid-impermeable, electrically conductive plates, commonly referred to as the anode and the cathode plates, respectively. The plates are typically formed from graphite, a graphite composite such as graphite/epoxy, but can also be formed from other suitable electrically conductive materials. The plates serve as current collectors, provide structural support for the porous, electrically conductive electrodes, provide means for carrying the fuel and oxidant to the anode and cathode, respectively, and provide means for removing water formed during operation of the fuel cell. When the channels are formed in the anode and cathode plates, the plates are referred to as fluid flow field plates. When the anode and cathode plates overlay channels formed in the anode and cathode porous material, the plates are referred to as separator plates.
Reactant feed manifolds are generally formed in the anode and cathode plates, as well as in the MEA, to direct the fuel (typically a substantially pure hydrogen gas stream or hydrogen-containing reformate gas stream from the conversion of hydrocarbons such as methanol or natural gas) to the anode and the oxidant (typically substantially pure oxygen or oxygen-containing gas) to the cathode via the channels formed in either the fluid flow field plates or the electrodes themselves. Exhaust manifolds are also generally formed in the anode and cathode plates, as well as the MEA, to direct the unreacted components of the fuel and oxidant streams, as well as water accumulated at the cathode, from the fuel cell.
Multiple fuel cell assemblies comprising two or more anode plate/MEA/cathode plate combinations, referred to as a fuel cell stack, can be connected together in series (or in parallel) to increase the overall power output as required. In such stack arrangements, the cells are most often connected in series, wherein one side of a given fluid flow field or separator plate is the anode plate for one cell, the other side of the plate is the cathode plate for the adjacent cell, and so on.
Perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its NAFION trade designation, have been used effectively in electrochemical fuel cells. Fuel cells employing Perfluorosulfonic cation exchange membranes require accumulated water to be removed from the cathode (oxidant) side, both as a result of the water transported across the membrane with cations and product water formed at the cathode from the electrochemical reaction of hydrogen cations with oxygen. An experimental perfluorosulfonic ion exchange membrane, sold by Dow Chemical Company under the trade designation XUS 13204.10, appears to have significantly less water transported with hydrogen cations across the membrane. Fuel cells employing the Dow experimental membrane thus tend to accumulate less on the cathode (oxidant) side, as the accumulated water at the cathode is essentially limited to product water formed from the electrochemical reaction of hydrogen and oxygen.
Recently, efforts have been devoted to identifying ways to operate electrochemical fuel cells using other than pure hydrogen as the fuel. Fuel cell systems operating on pure hydrogen are generally disadvantageous because of the expense of producing and storing pure hydrogen gas. In addition, the use of liquid fuels is preferable to pure, bottled hydrogen in mobile and vehicular applications of electrochemical fuel cells.
Recent efforts have focused on the use of an impure hydrogen fuel stream obtained from the chemical conversion of hydrocarbon fuels to hydrogen and carbon byproducts. However, to be useful for fuel cells and other similar hydrogen-based chemical applications, hydrocarbon fuels must be efficiently converted to relatively pure hydrogen with a minimal amount of undesirable chemical byproducts, such as carbon monoxide.
Conversion of hydrocarbons to hydrogen is generally accomplished through the steam reformation of a hydrocarbon such as methanol in a reactor sometimes referred to as a reformer. The hydrogen-containing stream exiting the reformer is generally referred to as the reformate stream. The steam reformation of methanol is represented by the following chemical equation: EQU CH.sub.3 OH+H.sub.2 O+heat.revreaction.3H.sub.2 +CO.sub.2 ( 1)
Due to competing reactions, the initial gaseous mixture produced by steam reformation of methanol typically contains about 65% to about 75% hydrogen, about 10% to about 25% carbon dioxide, as well as from about 0.5% to about 20% by volume of CO, all on a dry basis (in addition, water vapor can be present in the gas stream). The initial gas mixture produced by the steam reformer can be further processed by a shift reactor (sometimes called a shift converter) to reduce the CO content to about 0.2%-2% by volume, on a dry basis. The catalyzed reaction occurring in the shift converter is represented by the following chemical equation: EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 ( 2)
Even after a combination of steam reformer/shift converter processing, the product gas mixture will have minor amounts of CO and various hydrocarbon species, typically about 5% or less by volume, on a dry basis, of the total product mixture.
In low-temperature, hydrogen-based fuel cell applications, the presence of CO in the inlet fuel stream, even at the 0.1% to 1% level, is generally unacceptable. In solid polymer electrolyte fuel cells, the electrochemical reaction is typically catalyzed by an active catalytic material comprising a noble metal such as platinum. Carbon monoxide adsorbs preferentially to the surface of platinum, particularly at temperatures below about 150.degree. C., effectively poisoning the catalyst, and significantly reducing the efficiency of the desired electrochemical hydrogen oxidation reaction. A steam reformer/shift converter process can be used to reduce the amount of CO in the hydrogen-containing reformate gas stream to less than about 100 parts per million (ppm). In order to employ such a CO-containing reformate stream as the fuel stream for a fuel cell, the fuel cell must first be able to handle (i.e., the catalyst present in the MEAs cannot be poisoned by) the CO present in the reformate stream. In addition to the CO content of the reformate stream, CO can also be produced in the fuel cell by the reverse water shift reaction: EQU CO.sub.2 +H.sub.2 .revreaction.CO+H.sub.2 O (3)
In typical reformate fuel streams, the equilibrium concentration of CO from this reaction is about 100 ppm near room temperature.
The present method and apparatus oxidizes the carbon monoxide present in the incoming reactant stream of a fuel cell and/or produced by the reverse water shift reaction (reaction (3) above). The oxidation of carbon monoxide is particularly important where the electrocatalyst promotes the reverse water shift reaction, as is the case with platinum-containing catalysts.
Watkins et al. Canadian Patent No. 1,305,212 entitled "Method for Operating a Fuel Cell on Carbon Monoxide Containing Fuel Gas" discloses the oxidation of carbon monoxide present in a fuel gas introduced to a low-temperature, solid polymer electrolyte fuel cell which employs a noble metal catalyst, such as platinum, rhodium or ruthenium, in the anode. The method involves (a) reacting the fuel gas with an oxygen-containing gas, (b) contacting the resulting fuel gas mixture with a suitable catalyst to selectively convert carbon monoxide to carbon dioxide and thereby reduce carbon monoxide levels in the fuel gas to trace amounts, and (c) feeding the resulting substantially carbon monoxide-free fuel gas to the fuel cell.
Gottesfeld U.S. Pat. No. 4,910,099 entitled "Preventing CO Poisoning In Fuel Cells" discloses the injection of oxygen (O.sub.2) into the fuel stream, before introducing the fuel stream to the fuel cell, in order to remove CO present in the reformate fuel stream fed to the fuel cell. The oxygen so injected is in the form of either substantially pure O.sub.2 or oxygen-containing air.
Watkins' selective oxidation of carbon monoxide to carbon dioxide and Gottesfeld's injection of oxygen into the reformate fuel stream prior to introducing the fuel stream to the fuel cell, both effectively remove CO initially present in the fuel stream. However, the removal of CO upstream of the fuel cell will not affect the further production of CO within the reactant fuel stream of the fuel cell by the reverse water shift reaction. In this regard, the removal of CO from the fuel stream by selective oxidation and/or the initial injection of oxygen, will promote the production of CO by the reverse water shift reaction to produce CO (i.e., reaction (3) above will be driven to the right) because of the substantial presence of carbon dioxide and hydrogen in the fuel stream, as well as the presence of the platinum electrocatalyst in the fuel cell. In order to effectively remove CO produced in the reactant stream of the fuel cell, oxidant (either substantially pure oxygen or oxygen-containing air) should be introduced, preferably in a substantially uniform manner, across the active area of the fuel cell in which electrocatalyst is present. The uniform introduction of oxidant is particularly effective for fuel cell designs having large active areas and in which the residence time of the reformate stream in the fuel cell is prolonged.
Even in the absence of the reverse water shift reaction, the uniform introduction and distribution of oxygen across the active area of the fuel cell is advantageous. In this regard, the even introduction and distribution of O.sub.2 across the active area of the fuel cell promotes the maintenance of a uniform temperature profile across the active area by preventing temperature increases from the oxidation reactions (reactions (1) and (2) above). A uniform temperature profile in turn prevents the localized heating and sintering of the catalyst. Catalyst sintering can reduce the surface area of the catalyst, inhibit the mass transport through the catalyst, and lower the porosity of the catalyst, thereby diminishing the ability of the catalyst to promote the desired electrochemical reactions in the fuel cell. Thus, the uniform introduction and distribution of oxygen into the active area of the fuel cell not only effects the oxidation of carbon monoxide, but also maintains an advantageous uniform temperature profile across the active area.
Accordingly, it is an object of the present invention to provide a method and apparatus for reducing the concentration of carbon monoxide in a hydrogen-containing gas mixture so as to render the mixture suitable for use as the fuel stream for electrochemical fuel cells, and for other applications employing catalysts that would be adversely affected by higher carbon monoxide concentrations.
It is also an object of the invention to provide a method and apparatus for the oxidation of carbon monoxide to carbon dioxide in a reactant stream within an electrochemical fuel cell.
Another object of the invention is to provide an apparatus and a method for the oxidation of carbon monoxide, produced by the reverse water-shift reaction in a hydrogen-containing reformate gas mixture, by introducing oxygen or an oxygen-containing gas mixture at locations along the reaction pathway within a fuel cell.
A further object of the invention is to provide a method and apparatus for the oxidation of carbon monoxide in a hydrogen-containing reformate gas mixture by introducing oxygen or an oxygen-containing gas mixture at various locations along the reaction pathway in the active area of a fuel cell.
A still further object of the invention is to provide a method and apparatus for the uniform introduction and distribution of oxygen or an oxygen-containing gas mixture into the active area of the fuel cell to maintain a uniform temperature profile across the active area.